The present invention relates to a pneumatic radial tire having improved durability of the belt portion and particularly relates to a heavy-load pneumatic radial tire adapted particularly for use in trucks or buses.
For example, in a pneumatic radial tire for use in buses or trucks, the belt layer of the tread portion usually has a four layer structure, and the cords of a second belt layer and those of a third belt layer, in the direction of from a carcass layer to a tread of the tire, cross one another respectively at an angle of 10.degree. to 30.degree. to the circumferential direction of the tire (these second and third belt layers are collectedly referred to as a "cross-ply layer"). This cross-ply layer is a belt layer capable of substantially functioning as a tension band which mainly plays a role of bearing load, internal pressure, etc.
In the above-described cross-ply layer, the second and third belt layers are generally made of the same kind of wire cord in the same end count. However, after the completion of the primary life (i.e., after use of a fresh tire), the fatigue proportion of the cords constituting the second belt layer is larger than that of the cords constituting the third belt layer, which brings about a lowering in the life of the whole cross-ply layer. This unfavorably brings about a lowering in the durability of the belt portion.